(1) Field of the Invention
The present invention relates to motors driven using inverters without smoothing circuit and matrix converters, and particularly to a rotary drive mechanism of a motor which shows no pulsation caused by reduced rotations and reduced torque of the motor even when the motor is driven on a single-phase 100 V used in typical household.
(2) Description of the Related Art
FIG. 7A and FIG. 7B show voltage waveforms that show relationship between voltage and time on single-phase power supply and three-phase power supply. In FIG. 7A and FIG. 7B, a vertical axis represents the voltage and a horizontal axis represents the time.
In the case of three-phase power supply shown in FIG. 7A, each of three phases has phase difference of 120 degrees with respect to the other phases. For example in Japan, three phases vary in the range from −140 V to +140 V at 50 Hz or 60 Hz.
As shown in FIG. 7A, in the case of three-phase power supply, a matrix converter can provide −140 Vp with a fluctuation of 15% or less as well as +140 Vp having a fluctuation of 15% or less by selecting an envelope having the largest voltage. Here, the Vp indicates a half of a voltage value which is a width between a maximum value and a minimum value in a voltage waveform. Thus, a direct-current component having a little pulsation can be obtained by switching the phases as necessary, and smoothing by a capacitor is not required.
In single-phase power supply, there are two phases each having a 180-degree phase difference with respect to each other. As shown in FIG. 7B, selecting a voltage envelope cannot eliminate a zero crossing. As a result, even with the matrix converter, voltage reaches 0 V at every half cycle of 50 Hz or 60 Hz unless smoothing by a capacitor is performed, causing a problem of pulsation caused by reduced rotations and reduced torque of a motor.
Meanwhile, to achieve downsizing of inverters, inverters without smoothing circuit that have smaller smoothing capacitors are beginning to be used. When single-phase 100 V power supply provided to typical households is used, sufficient smoothing is not performed by the capacitor. Thus, the voltage output after diode rectification is not a direct-current component, leading to a problem of generation of large pulsation.
In other words, since an alternating-current component is outputted as it is, output voltage approaches 0 V at every half cycle of 50 Hz or 60 Hz. This poses a problem of not only noise in audible field and vibration but also pulsation caused by reduced rotations and reduced torque of a motor. In addition, there is a problem that motor can stop running.
In view of the above, there is a demand for motors which continuously rotate even when the output voltage becomes 0 V at every half cycle of 50 Hz or 60 Hz.
In related art, there is a Permanent Magnet synchronous motor (PM synchronous motor) in which variation in number of rotations is suppressed (Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2002-320371).
As shown in FIG. 5, when three-phase alternating-current power supply is used, a capacitor is not necessary, energy efficiency is high, variation in torque is small and thus vibration is not generated. Thus, an objective is to provide a PM synchronous motor 51 which is suitable for low speed rotation maintaining the same number of rotations, even when variation in a voltage or a load occurs, and can be used on both direct current (DC) power supply and alternating-current (AC) power supply.
To suppress variation in number of rotations, the PM synchronous motor 51 includes: a rotor 52 which includes a permanent magnet 52c; and three stators 53 each of which includes a solenoid coil 53a that is interposed between the two yokes 53b and 53c that include pole teeth 53d and 53e. The three stators 53 each having an electrical phase angle shifted by 120 degrees are disposed on an outer circumference of the rotor 52. A control device that includes a three-phase full-wave driver, a control signal generator and an oscillator which includes variable unit is formed, and a rotary drive mechanism driven on the DC power supply is thus provided. Further, the rotary drive mechanism includes a control device which performs control using three-phase AC power supply in which a frequency of the PM synchronous motor 51 is variable.
FIG. 6 describes an embodiment of a single-phase/three-phase conversion device (matrix converter) (Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2005-160257). An objective of this embodiment is to provide a control device for a single-phase/three-phase conversion device such that pulsation generated by single-phase instantaneous power can be compensated.
An AC power storage circuit PS for storing AC power is provided between three connection points P1 to P3 in three series switching circuits and a common connection point CP2. The power storage circuit PS stores a pulsation component of single-phase AC power. The AC power storage circuit PS that includes an AC reactor Ls, which is disposed between three connection points P1 to P3 in three series switching circuits and the common connection point CP2, and a switching circuit SWC is formed.
The switching circuit SWC selectively connects the AC reactor Ls to one of the three connection points P1 to P3 in three series switching circuits such that all the pulsation components of single-phase AC power are converted into instantaneous power that is provided to the AC reactor L. Although this embodiment allows single-phase driving, a smoothing capacitor and three reactors are required.